Add Parse Server option auto#OnLogin to automatically create a user on log-in if none exists

[mtrezza]

New Feature / Enhancement Checklist

• Report security issues confidentially.
• Any contribution is under this license.
• Before posting search existing issues.

Current Limitation

If a user is created with auth data, there is a single linkWith request that logs in an existing user, or if needed automatically creates a new user. Using email and password requires two separate requests, for example a log-in attempt, and if no user was found, signing up, or vice-versa.

Feature / Enhancement Description

Add new Parse Server option auto#OnLogin that short-circuits the login endpoint to automatically create a user if none exists yet.

[parse-github-assistant]

Thanks for opening this issue!

• 🎉 We are excited about your ideas for improvement!

[sahil-ansari01]

Interested in this issue, I'll push a pull request soon after fixing the issue!